1. Field of the Invention
The present invention relates generally to multiple access, spread spectrum, communication systems and networks. More particularly, the present invention relates to increasing user access capacity in a spread spectrum communication system.
2. Related Art
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as those used in code division multiple access (CDMA) communication systems provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. Such techniques are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters," and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, under the title "Method and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energy," both of which are incorporated herein by reference.
The above-mentioned patents disclose multiple access communication systems in which a large number of generally mobile or remote system users each employ at least one transceiver to communicate with other system users or users of other connected systems, such as a public telephone switching network. The transceivers communicate through gateways and satellites, or terrestrial base stations (also sometimes referred to as cell-sites or cells).
Base stations cover cells, while satellites have footprints (also referred to as "spots") on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into "sectors" by using directional antennas at the base station. Similarly, a satellite's footprint can be geographically divided into "beams," through the use of beam-forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be assigned multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite systems, each CDMA channel is referred to as a "sub-beam," because there may be several of these per "beam."
In communication systems employing CDMA, separate links are used to transmit communication signals to and from a gateway or base station. A forward link refers to the base station- or gateway-to-user terminal communication link, with communication signals originating at the gateway or base station and transmitted to a system user, or users. A reverse link refers to the user terminal-to-gateway or -base station communication link, with communication signals originating at a user terminal and transmitted to the gateway or base station.
The reverse link is comprised of at least two separate channels: an access channel and a reverse traffic channel. Generally, there are several access and reverse link traffic channels in a communication system. An access channel is used by one or more user terminals, separated in time, to initiate or respond to communications from a gateway or base station. Each such communication process is referred to as an access signal transmission or as an "access probe." The reverse traffic channels are used for the transmission of user and signaling information or data from user terminals to one or more gateways or base stations during a "call" or communication link setup. One structure or protocol for access channels, messages, and calls is illustrated in more detail in the Telecommunications Industry Association IS-95 standard entitled "Mobile Station-Base-Station Compatibility Standard For Dual-Mode Wideband Spread Spectrum Cellular System," which is incorporated herein by reference.
In a typical spread-spectrum communication system, one or more preselected pseudo-noise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user terminal communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell or sub-beam. In some communication systems, the same set of PN spreading codes are used in the reverse link for both the reverse traffic channels and the access channels. In other proposed communication systems, the forward link and the reverse link use different sets of PN spreading codes.
Generally, the PN spreading is accomplished using a pair of pseudonoise (PN) code sequences to modulate or "spread" information signals. Typically, one PN code sequence is used to modulate an in-phase (I) channel while the other PN code sequence is used to modulate a quadrature-phase (Q) channel in a technique commonly referred to as quadrature phase-shift keying (QPSK). The PN spreading occurs before information signals are modulated by a carrier signal and transmitted from the gateway or base station to the user terminal as communication signals on the forward link. The PN spreading codes are also referred to as short PN codes because they are relatively "short" when compared with other PN codes used by the communication system. Typically, the same set of PN spreading codes are shared by the forward and reverse link traffic channels and another set of PN spreading codes are used for the access channels as discussed above.
A particular communication system may use several lengths of short PN codes depending on whether the forward link or the reverse link channels are being used. In the forward link, such as in a satellite system, the short PN codes typically have a length from 2.sup.10 to 2.sup.15 chips. These short PN codes are used to discriminate between the various signal sources, such as gateways, satellites, and base stations. In addition, timing offsets within a given short PN code are used to discriminate between beams of a particular satellite, or cells and sectors in terrestrial systems.
In a proposed satellite communication system, the short PN codes used in the reverse link have a length on the order of 2.sup.8 chips. These short PN codes are used to enable a gateway or base station receiver to quickly search out user terminals that are trying to access the communication system without the complexity associated with the "longer" short PN codes used in the forward link. For purposes of this discussion, "short PN codes" refer to these short PN code sequences (2.sup.8) to be used in the reverse link.
Another PN code sequence, referred to as a channelizing code, is used to discriminate between communication signals transmitted by different user terminals on the reverse link within a cell or sub-beam. The PN channelizing codes are also referred to as long codes because they are relatively "long" when compared with other PN codes used by the communication system. The long PN code typically has a length on the order of 2.sup.42 chips, but may be shorter or masked as desired. Typically, an access message is modulated by the long PN code prior to being modulated by the short PN code and subsequently transmitted as an access probe or signal to the gateway or base station. However, the short PN code and the long PN code may be combined prior to modulating or spreading the access message.
When a receiver at the gateway or base station receives the access probe, the receiver must despread the access probe to obtain the access message. This is accomplished by forming hypotheses, or predictions, as to which long PN codes and which short PN code pair were used to modulate the access message. A correlation between a given hypothesis and the access probe is generated to determine which hypothesis is the best estimate for the access probe. The hypothesis that produces the greatest correlation, generally relative to a predetermined threshold, is selected as a hypothesis of the most likely code and timing match. Once the selected hypothesis is determined, the access probe is despread using the selected hypothesis to obtain the access message.
In a communications system having many users, it is likely that more than one access probe will arrive at a gateway or base station simultaneously, or within a preselected period of time over which the signal is to be detected. When this happens, the access probes can collide or mutually interfere, rendering them unrecognizable to the gateway or base station. One way to avoid such collisions is to employ a centrally-controlled access technique, where the communications system schedules user terminal access probe transmissions. One disadvantage of such a technique is that a significant amount of access channel bandwidth is consumed by such a scheduling mechanism.
Another technique used to avoid such collisions is the slotted random access technique, such as the "slotted ALOHA" technique. In the slotted random access technique, a regular system-wide timing structure establishes permissible transmission or reception times. The access channel is usually divided into a series of fixed length frames or time "slots" or windows, each having the same fixed duration slots used for receiving signals. The access signals are generally structured as "packets", that consist of a preamble and a message portion, that must arrive at the beginning of a time slot to be acquired. A user terminal transmits at its own discretion, but is constrained to transmit only within the boundaries of a single slot to have a message received. The use of this technique on the access channel significantly decreases the possibility that access probes from different users will collide at a gateway or base station.
Unfortunately, the slotted random access technique also results in a significant amount of unused time on the access channel. Because an access probe must be transmitted within a single slot, the slot duration must be chosen to exceed the duration of the longest possible access probe. Because all slots are of the same duration, a slot will be partially empty for all but the longest access probe. The result is a substantial amount of wasted bandwidth on the access channel and a consequent reduction in the user capacity of the access channel.
A failure to acquire an access probe during a particular frame period results in the transmitter desiring access having to re-send the access probe to allow the receiver to detect the probe again during a subsequent frame. Multiple access signals arriving together "collide" and are not acquired, requiring both to be resent. In either case, the timing of subsequent access transmissions when the initial attempt fails is based on a delay time equal at a minimum to the length of the time slots, and generally to a random number of time slots or frames. Therefore, a significant amount of time passes before an access probe can again be resent and received. The length of the delay in probe acquisition is increased by any delay in resetting acquisition circuits in the receiver to scan the various hypothesis, and in other probes being acquired first, as mentioned. Ultimately, the access probe may never, at least not within a practical time limit, be acquired if the timing uncertainty is not resolved.
What is needed is a system and method for increasing user capacity on a slotted random access channel in a spread spectrum communication system. It is preferable that the technique allow access probes to be received with minimum delay and efficiency.